Thin-film photovoltaic cells typically use semiconductors such as CdTe or copper indium gallium sulfide/selenide (CIGS) as an energy absorber material. Due to the limited availability of indium, alternatives to CIGS are sought. Kesterite (Cu2ZnSnS4 or “CZTS”) possesses a band gap energy of about 1.5 eV and a large absorption coefficient (approx. 104 cm−1), making it a promising CIGS replacement. In addition, CZTS contains only non-toxic and abundant elements.
Current techniques to make CZTS thin films (e.g., thermal evaporation, sputtering, hybrid sputtering, pulsed laser deposition and electron beam evaporation) require complicated equipment and therefore tend to be expensive. Electrochemical deposition is an inexpensive process, but compositional non-uniformity and/or the presence of secondary phases prevents this method from generating high quality CZTS thin films. CZTS thin films can also be made by the spray pyrolysis of a solution containing metal salts, typically CuCl, ZnCl2, SnCl4, and thiourea as the sulfur source. This method tends to yield films of poor morphology, density and grain size. Photochemical deposition has also been shown to generate p-type CZTS thin films. However, the composition of the product is not well controlled, and it is difficult to avoid the formation of impurities such as hydroxides. Quaternary CZTS precursor powders can be prepared and deposited on a substrate by standard printing techniques. Subsequent annealing in a nitrogen and sulfur atmosphere leads to the formation of CZTS films. However, it is difficult to control the molar ratio of elements in the CZTS powder, which limits the ultimate performance of the CZTS thin film.
The formation of kesterite from uncoated binary and ternary sulfides has also been disclosed.
However, there still exists a need for a process that provides high quality CZTS thin films at a low cost.